Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q27: Focus Session: Electron-hole Interaction in Nanoparticles |
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Sponsoring Units: DCOMP Chair: Ari Chakraborty, Syracuse University Room: 501 |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q27.00001: Excitation gaps of finite-sized systems from Optimally-Tuned Range-Separated Hybrid Functionals Invited Speaker: Leeor Kronik Excitation gaps are of much significance in electronic structure theory. Within many-body perturbation theory, the fundamental gap is the difference between the lowest quasi-hole and quasi-electron excitation energies and the optical gap is addressed by including the quasi-electron - quasi-hole interaction. A long-standing challenge has been the attainment of a similar description within density functional theory (DFT), with much debate on whether this is achievable even in principle. Here, I describe a new DFT approach to this problem. Anchored in the generalized Kohn-Sham framework, our method is based on a range-split hybrid functional with exact long-range exchange. Its novel feature is that the range-splitting parameter is determined from first principles, per-system, based on satisfaction of physical constraints. For finite objects, this approach mimics successfully the quasi-particle excitation picture. It allows the extraction of the fundamental and optical gap from one underlying functional, based on the ground-state HOMO-LUMO gap and the lowest excitation of linear-response time-dependent DFT, respectively. It is equally accurate for the difficult case of charge-transfer excitations and it produces accurate outer-valence simulated photoelectron spectra. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q27.00002: First-Principles Investigation of Hot Carrier Relaxation in Quantum-Dots Kyle Reeves, Lesheng Li, Yosuke Kanai A combination of fewest-switches surface hopping simulations and Kohn-Sham (KS) density functional theory allows us to obtain key insights into hot carrier relaxation processes in materials. In particular, a first-principles approach allows us to investigate the influence of atomistic details such as the surface chemistry on the relaxation process in quantum dots. At the same time, the accuracy of the KS single-particle energies impacts the simulated non-adiabatic relaxation rate. Using a small gold nanocluster as a model system, we investigate how relaxation rates are influenced by the single-particle energies by considering a hybrid functional in DFT and the GW approximation. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q27.00003: Investigation of effect of particle size and heterojunction on electron-hole interaction in CdSe and CdSe/ZnS quantum dots Jennifer Elward, Jeremy Scher, Arindam Chakraborty The focus of this work is to explore both the effect of dot size and the effect of heterojunction on the electron-hole (e-h) interaction in nanoparticles. The exciton binding energy (EB), electron-hole recombination probability (P-eh) and average electron-hole separation (R-eh) are important metrics used to quantify the e-h interaction. The form of the wavefunction is critical for accurate representation of these properties. In this work, the explicitly correlated Hartree-Fock (XCHF) and explicitly correlated configuration interaction (XCCI) methods are used. The effect of dot size on e-h interaction was investigated by studying a CdSe quantum dot (QD) system, with diameters ranging from 1-20 nm. The EB and P-eh were found to be strongly dependent on dot size, however, the scaling of each property was distinctly different. The effect of heterojunction was explored by studying a CdSe/ZnS core/shell QD system. The effects of increasing shell thickness, core size and effect of volume versus effect of heterojunction were assessed by computing the EB, P-eh and R-eh. It was found that these properties are also dependent on size, however, the dependence of the CdSe/ZnS core/shell system was markedly different from the CdSe core-only system. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q27.00004: Electronic Properties of Defects at the Surface of Embedded Silicon Nanoparticles Nicholas Brawand, Marton Voros, Giulia Galli Using density functional theory calculations we predicted the single particle energies and lifetimes of dangling bond defects at the surface of various Si nanoparticles (NPs) embedded in amorphous SiO$_2$ matrices [1]. We found that both the lifetimes and the single particle excitation energies of these defects depend on the size of the NP. However, the energy positions of the dangling bond defect levels are always within the NP gap, irrespective of size, between 1 and 2 nm. Our findings suggest that the presence of silicon NPs within amorphous SiO$_2$ may impact the functionality of silicon-on-insulator nanophotonic devices operating near 1.5 $\mu$m [2]. [1] Li, Tianshu et al. Phys. Rev. Lett., 107, 206805 (2011) [2] Bogaerts, W. et al. Journal of Lightwave Technology, 23, 401-412 (2005) [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q27.00005: Quasiparticle band structures and thermoelectric transport properties of Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn Guangsha Shi, Emmanouil Kioupakis Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn are narrow-gap semiconductors with large Seebeck coefficients and favorable thermoelectric properties. We calculated the quasiparticle band structures of Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn using density functional and many-body perturbation theory in the GW approximation. The calculated band gaps are in good agreement with experiment. The inclusion of semicore states in the valence is necessary to obtain accurate band gaps for Mg$_{2}$Ge and Mg$_{2}$Sn. We used the maximally localized Wannier function method and the Boltzmann transport equation in the constant relaxation-time approximation to determine the Seebeck coefficient and the electrical and carrier thermal conductivities. We discuss the importance of quasiparticle corrections to accurately determine the Seebeck coefficients at high temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q27.00006: Optical, electronic and transport properties of tetrahedrites Simon Kohl, Jason Vielma, David Foster, Guenter Schneider Doped Tetrahedrites Cu$_{12-x}$TM$_{x}$Sb$_{4}$S$_{13}$ (TM={Fe,Mn,Zn}) have recently attracted interest as thermoelectric materials. We present an ab-initio study based on density functional theory of the optical, electronic and transport properties of these materials. We find in Cu$_{12-x}$Zn$_{x}$Sb$_{4}$S$_{13}$: 1. the band-gap can be tuned through chalcogenide substitution and the optical absorption is very large making tetrahedrites attractive also as solar absorber materials. A point defect study of the Zn rich tetrahedrite (x=2) based on supercell calculations indicates p-type conductivity and Cu-Zn antisite defects are the dominant acceptor defect with Cu-vacancies also contributing. The calculated hole concentration is much larger than what is expected from conductivity measurements. We discuss these results in the context of the observed unusual, variable range hoping like electronic transport properties. Finally we present results of thermopower calculations based on semiclassical Boltzmann theory and discuss the applicability of these approach for tetrahedrites. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q27.00007: Strong core hole in resonant inelastic x-ray scattering (RIXS) Robert Markiewicz, John Rehr, Arun Bansil We apply a lattice version of Mahan, Nozi{\`e}res, and de Dominicis theory$^1$ to RIXS calculations to understand the role of the core hole. The model reproduces the decomposition of the RIXS spectrum into well- and poorly-screened components. While the calculation can reproduce the full multiband spectrum, single pair excitations contribute the dominant part to the RIXS spectrum, and they can be described as the dynamic structure function $S(q,\omega )$ dressed by matrix element effects. We find evidence for an edge singularity at the RIXS threshold, similar to that found in x-ray absorption. We will discuss comparisons with long core hole lifetime calculations, and extensions to a system with antiferromagnetic order. 1. G.D. Mahan, Phys. Rev. {\bf 163}, 612 (1967); P. Nozi\`eres and C.T. De Dominicis, {\it ibid}. {\bf 178}, 1097 (1969). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q27.00008: Thermoelectric properties of titanium dioxide polymorphs from first principles Dylan Bayerl, Emmanouil Kioupakis Titanium oxides are promising materials for high-temperature thermoelectrics because of their high Seebeck coefficients, thermal stability, and natural abundance. We use first-principles calculations to investigate the thermoelectric transport properties of several titanium dioxide polymorphs. Our methodology is based on density functional and many-body perturbation theory within the GW approximation. The maximally localized Wannier function method is employed to interpolate the GW bands in the Brillouin zone. We use the Boltzmann transport formalism within the constant relaxation time approximation to determine the temperature and carrier-density dependence of the Seebeck coefficient, electron mobility, and electron thermal conductivity from the calculated electronic band structures. We demonstrate agreement with experimentally measured transport parameters and enhanced power factor at high temperature in certain heavily doped phases. This research was supported as part of CSTEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Computational resources were provided by the DOE NERSC facility. [Preview Abstract] |
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